1. Field of the Invention
This invention relates to a resilient pump packing for reciprocating pumps, and more particularly it relates to a generally cup-shaped pump packing which provides resistance to deterioration and wear and consequent long useful life.
2. Description of the Prior Art
It has long been desirable to have packings for piston type pumps which provide constant sealing, reduced friction between the packing and the cylinder wall around the packing, and long packing life. A slush pump which purports to provide these characteristics is disclosed in U.S. Pat. No. 2,677,581, issued to Taylor. The pump piston disclosed in Taylor includes a longitudinally extending circular lip which is formed by a tapered flange of resilient material which provides a tight fit when compressed within the pump cylinder. The packing in Taylor is backed up by a solid flange member and is therefore not called upon to support pumping pressure solely with resilient packing material.
Several U.S. patents disclose packings for obtaining fluid-tight seals which include embedded metallic spring members. Such a disclosure is seen in U.S. Pat. No. 2,723,721 issued to Corsette which describes a packing to obtain a fluid-tight seal between a pipe and the wall of an uncased well hole or between a pipe and the casing in a well hole. Two rows of flat springs are concentrically arranged, so that they are fixed at one end and staggered in position around the circumference of the packing. The flat springs are embedded in the packing so that no packing is extruded due to high pressures on the inside surface of the packing. The Corsette disclosure does not discuss a predetermined pressure profile between the packing and the surrounding cylinder. Corsette refers to a tapered surface at the packing lip together with a rounded end, which preclude concentration of pressure at the packing lip.
U.S. Pat. Nos. 3,172,799 to Waldrop and 3,346,267 to Farley, disclose swabs or packing cups containing embedded metal reinforcing members. The swab cups are useful in withdrawing fluids from oil well casings and are specifically constructed to have beveled or tapered leading edges so that they may be drawn through a series of joined casings or pipes without becoming jammed within the casings or pipes due to variations in pipe diameter occurring at the joints.
U.S. Pat. No. 3,819,192 issued to Berry et al discloses a reinforcing structure for use in a well pressure sealing cup. The reinforcing structure includes a body with a plurality of upstanding tines which is made from a polyfurcated plate which is rolled into a cylindrical shape and wherein the free ends of the tines are bent inwardly to guide the pressure sealing cup past joints and other irregularities within the pipe being sealed.
FIG. 1 of the drawing shows a typical prior art packing cup having a nitrile impregnated cotton duck body with a cylindrical external surface disposed adjacent to the open end of the cup adapted to engage a surrounding cylinder wall. The packing cup of FIG. 1 has been manufactured by the Hercules Products Division of the Richardson Company, Albany, New York, and others and is provided with a shape which fits in a typical packing holder to be retained therein by a packing retainer ring. The packing holder is attached to the pump piston rod so that the packing will be reciprocated within the cylinder wall. The arrow in FIG. 1 indicates the direction of movement of the packing during the fluid pressure half of its pumping cycle. Referring to the graph of FIG. 7 a line marked "prior art 1" is shown which depicts the pressure profile between the heel and the forward edge of the external cylindrical surface of the packing of FIG. 1. With a working pressure, PW, against the inner cupped surface of the packing and atmospheric pressure, PA, against the opposite side of the packing cup of FIG. 1, it is observed that contact pressure across the cylindrical wall-engaging surface includes one pressure spike behind the forward edge of the cup and a second pressure spike near the heel of the cylindrical surface. Such a contact pressure profile provides for undue wear near the heel of the cylindrical surface where it is in contact with the cylinder wall and also indicates "ballooning" of the body member in the region near the heel of the cylindrical surface, as is shown, due to the differential between PW and PA. More precisely the high pressure graident at the heel region for the packing of FIG. 1 does not allow a fluid film to be retained between the packing and the cylinder wall. The film is quickly squeezed out toward the low pressure side and the essential cooling and lubricating film is lost. "Ballooning" at the heel allows elastomer material to extrude into the clearance between the packing holder and cylinder wall, further raising contact pressure and causing rapid wear. Ballooning also is an indication of flexure which fatigues the elastomer and results in bursting failure of the packing. The packing of FIG. 1 is shown in the relaxed condition by dashed lines and in the installed condition by solid lines.
FIG. 2 of the drawings shows another well-known reciprocating pump packing which has been in use for many years in Bean piston pumps manufactured by FMC Agricultural Machinery Division, Jonesboro, Arkansas. The pump packing of FIG. 2 is a generally spherical cup which is formed to fit within a well-known packing holder. The packing is held within the holder by a packing retainer ring as shown. The packing of FIG. 2 is shown in its relaxed shape by dashed lines. The lip of the packing of FIG. 2 is substantially compressed to take the form shown by solid lines when fitted within the wall of a pump cylinder. Typically a 0.125 inch radial interference fit is provided between the packing radius and the cylinder radius. The packing of FIG. 2 is shown to produce a ballooned portion near the forward edge of the rigid packing holder in much the same manner as described in conjunction with the prior art packing of FIG. 1. A corresponding pressure profile indicated as "prior art 2" in FIG. 7 shows a high pressure zone appearing at that point in the contact region between the cylinder wall and the packing. In the pressure profile of FIG. 7 substantially no contact pressure is seen between the forward lip of the packing and the cylinder wall. This situation creates a severe wear problem in the heel portion of the contact region due to the localized high pressure on the packing material there and the possibility of migration of foreign matter behind the packing cup lip and into the region between the packing cup and the cylinder wall. The wear problem is intensified by the flexing of the packing material due to the aforementioned "ballooning" of the packing cup, as shown, which occurs in the unsupported packing wall just forward of the packing holder. The flexing and "ballooning" results from the pressure differential between PW and PA and from the alternating axial friction forces applied to the packing by the cylinder wall during alternate half cycles of the pumping stroke.
It may readily be seen that a packing cup with support within the cup wall to reduce flexing therein and with a predetermined pressure profile between the cylinder wall and the wall-engaging surface of the packing cup would be desirable from the standpoint of decreasing wear and internal packing material deterioration and thereby increasing packing cup life.